Justin Bowden, the GMB union's national secretary for energy, said the decision to suspend work on Wylfa Newydd in Anglesey raised "the very real prospect of a UK energy crisis".
"While the government has had its head up its proverbial backside over Brexit, vital matters like guaranteeing the country's future energy supply appear to have gone by the wayside."
Hitachi has said it will suspend work on a multi-billion-pound UK nuclear project because of rising costs.
The decision puts thousands of jobs at risk if the £13bn plant at Wylfa Newydd in Anglesey, north Wales, is scrapped.
The Japanese firm had been in talks with the UK government since June about funding for the project, which was being built by its Horizon subsidiary.
The government said it had failed to agree terms with Hitachi. The nuclear industry said it was "disappointing".
Hitachi said it would also suspend work on another site, in Oldbury in Gloucestershire, "until a solution can be found".